1. Field
The present invention relates to an electrochromic display that displays images by utilizing an electrochromic phenomenon.
2. Description of the Related Art
Electronic paper is getting more and more attention as a new display medium that combines the characteristics as paper, which is no need of information holding energy, can be securely saved, easily read, quickly readable and so on, and the characteristics as an electronic display capable of rewriting information and so on.
Various types are known as a display principle in the electronic paper. For example, it is a microcapsule type electrophoretic display method where capsules enclosing positively and negatively charged black and white particles are moved between electrodes. Further, it is a twist ball method where the direction of spherical particles colored in black and white are controlled. These methods perform display by utilizing a physical phenomenon.
On the other hand, there is known a method that performs display by utilizing a chemical phenomenon. Among others, one utilizing an electrochromic phenomenon is known where voltage is applied between electrodes to cause coloring or deletion by oxidation-reduction reaction. This is described in Japanese Patent Laid-Open No. 2002-258327 publication, for example.
A drive circuit same as a drive circuit in a liquid crystal display is used for driving each electrode of an electrochromic display as it is also shown in Japanese Patent Laid-Open No. 2002-258327 publication. This is a constitution where one TFT is used for each electrode, a gate electrode of the TFT is connected to a gate wire, a source electrode of the TFT is connected to a source wire, and a drain electrode of the TFT is connected to a pixel electrode, in which a charge quantity is controlled by source voltage. If the liquid crystal type drive circuit is directly used in the electrochromic display, a long time is required to accumulate necessary charge quantity in the pixel electrodes for oxidation-reduction reaction because the circuit depends on current capability of a source driver and TFT. Therefore, to increase response speed of display, it is necessary to proceed oxidation-reduction reaction in the electrochromic phenomenon in higher speed, and it is preferable that switching TFTs be capable of flowing as large current as possible when it is turned to ON state because charge quantity necessary for oxidation-reduction reaction can be supplied in a short time. Consequently, it is the first object of the present invention to provide an electrochromic display of high-speed response and high definition where TFTs are formed as large as possible to increase ON current.
Further, since the size of TFT that can be formed in one pixel is limited even if the size of TFT is made larger to increase the response speed of the electrochromic display, improving effect of response speed is limited only by making the size of TFT larger. In addition, use of high performance TFT drivers cause an increase of manufacturing cost. Thus, the second object of the present invention is to provide an electrochromic display of higher response speed and high definition where the constitution of a driver for TFTs is devised.
Furthermore, as the drive circuit for driving each pixel of the electrochromic display, a drive circuit generally used in an organic EL display could be used as shown in Japanese Patent Laid-Open No. 2002-108252 publication. Its constitution is that each pixel has: a TFT connected to a gate wire and a source wire; and a TFT made up of a gate electrode connected to the drain electrode of the foregoing TFT, a source electrode connected to a power supply wire, and a drain electrode connected to a pixel electrode. Since the organic EL type drive circuit has only one power supply wire, charge can be saved in the pixel electrode but it is difficult to reduce charge. In other words, a display state can be changed from white display to black display, but it is difficult to change the display state from black display to white display. This is because means for changing electric potential of the power supply wire or the like is necessary for the case of saving charge and reducing charge. Consequently, the third object of the present invention is to provide an electrochromic display of high-speed response and high definition where writing and deletion to pixels can be easily performed and rewrite time is shortened.